Semiconductor devices, such as magnetic random access memory (MRAM) devices, use magnetic memory cells to store information. Information is stored in the magnetic memory cells as an orientation of the magnetization of a free layer in the magnetic memory cell as compared to an orientation of the magnetization of a fixed, i.e., reference, layer in the magnetic memory cell. The magnetization of the free layer can be oriented parallel or anti-parallel relative to the fixed layer, representing either a logic “1” or a logic “0.” The orientation of the magnetization of a given layer (fixed or free) may be represented by an arrow pointing either to the left or to the right. When the magnetic memory cell is sitting in a zero applied magnetic field, the magnetization of the magnetic memory cell is stable, pointing either left or right.
Typically, an MRAM device comprises a number of word lines and bit lines running above and below an array of magnetic memory cells. The application of a magnetic field from the word lines and bit lines can switch the magnetization of the free layer from left to right, and vice versa, to write information to a given magnetic memory cell.
Several types of MRAM devices exist. In one type, referred to as “Stoner-Wohlfarth MRAM,” a single-layer free layer is used to store information. Stoner-Wohlfarth MRAM is an older version of MRAM and is largely considered obsolete for most applications. In another type, referred to as “Toggle MRAM,” a multiple-layer free layer, i.e., having two coupled magnetic layers, is used to store information.
To accommodate ever-increasing data processing rates, MRAM devices have to exhibit faster switching times with nanosecond (ns) and picosecond (ps) scale operation being desired for some applications. For example, Bailey et al., “Control of Magnetization Dynamics in Ni81Fe19 Thin Films Through the Use of Rare-Earth Dopants,” IEEE TRANSACTIONS ON MAGNETICS, vol. 37, no. 4, pgs. 1749-54 (July 2001) (hereinafter “Bailey”), the disclosure of which is incorporated by reference herein, describes ps scale operation, i.e., a ps scale write cycle time, of Stoner-Wohlfarth MRAM devices and using doping to tune the magnetic dynamics thereof. The idea is to use rare earth dopants to increase the damping of the Stoner-Wohlfarth single-layer free layer. Increasing the damping reduces the number of oscillations the free layer makes each time it is switched. Since the total write cycle time includes this oscillation time, increased damping hence reduces the write cycle time. Therefore, in Bailey, damping is increased solely to reduce the write cycle time in single-layer free layer devices.
Stoner-Wohlfarth MRAM was replaced by Toggle MRAM because Stoner-Wohlfarth MRAM suffered from thermally activated half select errors. A half select occurs when a magnetic field pulse is applied to only a word line or only a bit line (a half select pulse). Magnetic memory cells need to survive many repeated half select pulses without accidentally switching. If the magnetic memory cell does switch, it is called a half select error. These errors are thermally activated in that they would not occur at zero temperature, but at room temperature the thermal energy encourages the errors to occur. Thermally activated half select errors are characterized by being proportional to the length of time for which the half select pulse is applied (around 10 ns). Doubling the pulse length doubles the number of errors. Toggle MRAM does not suffer from this type of error.
However, Toggle MRAM is susceptible to a different type of error, called a thermally activated dynamic half select error. The term “dynamic” refers to the fact that the number of errors is independent of the length of time for which the half select pulse is applied. Instead, dynamic errors depend on a rise and fall time of the pulse (around 0.5 ns). The faster the rise and fall time of the pulse, the more errors there are. This is a problem that is not present in Stoner-Wohlfarth MRAM.
Therefore, techniques that minimize, or eliminate, thermally activated dynamic half select errors in Toggle MRAM would be desirable.